Grinding mica



April 3, 1951 W. T. M DANIEL, JR, ETAL GRINDING MICA Filed Aug. 17, 1949 IN V EN TORS BYE%ZZM Patented Apr. 3, 1951 UNITED STATES PATENT OFFICE GRINDING MICA William T; McDaniel, Jr., Asheville, and John R.

Le Grand, Wilmington, N. 0., assignors to Tennessee Valley Authority Application August 17, 1949, Serial No. 110,822.

Mica, as it occurs in nature, is usually in the form of tough, elastic, thin sheets consolidated to form blocks, or the so-called mica books. These blocks or books are easily splittableon planes parallel tothe eminent cleavage of mica crystal pattern. A mica book of the thickness of an ordinary sheet of cardboard may be easily split into 20 or more sheets, and each of these sheets maybe further subdivided. Mica is also a relatively soft mineral. From these characteristics one might expect that mica would be easily broken up in grindingmachines to produce fine particles in the form of fine, thin flakes. This, however, is far from the truth, as mica is one of the most dinlcult materials to grind to fine sizes. Even the thinnest films or flakes oithis material are tough and elastic, have appreciable resiliency, absorb shock, and are so smooth that they slip over one another as if greased. Such conventional grinding machines as ball and pebble mills, jaw and roll crushers, and other forms of crushers' and grinders are Wholly inadequate to produce fine-sized mica. It is therefore obvious that a grinding process for mica must embody unusual features. Y

Mica is usually ground by one of two general types of processes, namely, dry-grinding and wetgrinding. In the dry-grinding processes rod mills, burr mills, cage disintegrators, and other types of grinding apparatus have been used. Whenground dry, mica heats up rapidly and loses its sheen and color as well as its elasticity. The resulting powder is rough and has little use except as'a filler.

Wet-ground mica is very different in its prop.-' erties. It is in the form of fine, thin flakes, has a bulk weight of less than 12 pounds per cubic foot when measured on a Scott volumeter, has a high sheen, and a color of from 65" to-70 per cent of standard magnesium oxide measured through a green filter on a Photovolt reflection meter. It has 'very high covering power when spread outand has agreas'y, unguent feel. Mica of' this type sells at several times as muchper ton as dry-ground mica and is used inv paint, coating compositions, and wallpaper pigments, and has many other commercial uses where its pe culiar characteristics are desirable. A

While a number of continuous processes for wet grinding mica have been proposed, none has been commercially successful. Practically all wet-v ground mica is produced at present in batch processes by the use of mullers. The mullers usually used are large tubs having wooden bottoms made up of wooden blocks with the end grain disposed vertically. Within these tubs heavy wooden roll-. ers revolve upon anaxle which rotates horizontally upon a vertical central point to reduce the mica to fine-particle size. Coarse mica is'added to the tub along with suificient water to make a stiii mass. The heavy rollers then are started and travel over the mica, which'bears the weight of the rollers. Plows attached to the rotary members continuously plow the mica into the path of the rollers. A roller traveling in this manner exerts crushing, frictional, and twisting forces on the mica. Water is added from time to time as the operator thinks expedient, and the water content of the mass is maintained in the vicinity of 45 per cent. The Whole operation is very carefully watched and supervised and is conducted empirically. Standard practice includes grinding such a batch for about 8 hours. t

The energy'required, in relation to the fine mica produced, is usually about 1 horsepower-hour for each 5 to 7 pounds of minus 325-mesh mica produced. Evaporation of water by frictional heat often causes the mica to become dry and to lose its sheen because of the lack of water lubrication. This condition is combatted by the addition of extra water and by slowing the speed at which the roller rotates. At the completion of a grinding period the mica is removed from the tub into settling troughs or other separation equipment whereby the fine-sized mica is separated from the coarser material. A portion of the coarser material is usually returned to the muller for regrinding. However, acharge to the muller cannot contain very much of this relatively fine material because it acts as a lubricant on still coarser material and reduces the efficiency of grinding. Such a process requires a great deal of supervision and labor, is slow and expensive, and is unsuit ablefor production of large quantities of product.

Mica grinding has been considered an art and has been so practiced for years. Many of the steps and refinements ofprocesses'have been-held as trade secrets. Each steplis usually controlled in an entirely empirical manner, and the success depends to agreat extent upon the judgment and skill of the operator. Excellent descriptions of mica-grinding art and the mullerprocess have been published by Paul M. Tyler of the United States Bureau of Mines, Transactions of the American Institute of Mining and Metallurgical Engineers, Industrial Minerals Division (Non- Metallics), vol. 148, 105-21 (1942) and by Dent et al. in Engineering and Mining Journal, June 1948 (McGraw-Hill).

Among the many disadvantages of muller-type mica grinding are the high power requirements per pound of finished product, the slowness of the grinding and necessity for use of batch-type processes involving much hand labor, the requirement for high skill and empirical judgment of operators, and the inability to use much fines as charge to the muller. The last disadvantage has, until now, prevented the use of mica recovered from mica schist by flotation processes as charge to mullers or any other method of producing water-ground mica. Very large deposits of mica schist exist in North Carolina, Georgia, and in other locations, but have hitherto been useless because the mica recovered could not be processed into the high-grade, wet-ground mica.

It is an object of this invention to provide a method for the production of wet-ground mica which is adapted to utilize mica feed containing high proportions of small-sized mica.

Another object is to provide a method for the production of wet-ground mica which is adapted to utilize mica recovered from mica schist by flotation methods.

Another object is to provide a method for the production of wet-ground mica in which the requirement of power per pound of fine product is greatly reduced.

Another object is to provide a method for the production of wet-ground mica which requires less skill and empirical judgment on the part of operators than is necessary in processes employing mullers.

Another object is to provide a method for the production of wet-ground mica which may be made continuous.

Other objects and advantages will become apparent as this disclosure proceeds.

We have now found that these objects may be attained by use of the novel grinding process described below.

Briefiy, this process comprises moistening mica with water to form a stiff, nonfluid mass, containing from about 65 to about 85 per cent mica, i. e., from about 35 to about per cent water, introducingthe resulting moistened mica into a disintegration zone, maintaining pressure of at least 2 p. s. i. g. and preferably in the range from about 3 to about 11 p. s. i. g. on the moistened mass in the disintegrating zone while subjecting the mass to intensive agitation therein for periods of time in the range from about seconds to about 3 minutes.

In the attached drawing, Figure 1 shows one type of apparatus in which our novel process may be carried out continuously. Figure 2is a vertical section of another type of apparatus in which our novel process may be conducted batch- Wise. Figure 3 is a plan view of one of the many types of impellers or agitators which may be used in either apparatus.

When our novel process is carried out continuously in an apparatus shown in Figure 1, mica containing from 15 to weight per cent of water is introduced via opening 5 into the interior of hollow cylinder 6. The moistened mica is forced by revolution of the screw '1 into a disintegration zone 8. Here the mica is strongly agitated by rotation of impeller 9 driven by the shaft I0. Disintegrated mica is constantly forced out of the disintegration zone 8 by fresh incoming feed and passes into the interior of elastic member I I, which is constricted by an adjustable band 12. The rate of introduction of feed, rate of rotation of screw 1, and the degree of constriction imposed by band [2 are adjusted to maintain a pressure of at least 2 p. s. i. g. and preferably in the range from about 3 to about 11 p. s. i. g. on the mica in the disintegration zone, and also to ensure that mica passing through the disintegration zone is retained therein for a period from about 30 seconds to about 3 minutes. Ground mica is continuously discharged from the opening I3 of elastic member 1!. Band 12 is adapted to constrict the elastic member ll so as to maintain back pressure on the mica and keep the disintegration zone full of mixture at all times, control the retention time of the mixture within disintegration zone 8, and to control the pressure within the agitator.

Figure 2 illustrates another form of apparatus in which our invention may be carried out batchwise. Mica containing from about 15 to about 35 per cent of water is introduced into disintegration vessel 2%. Vessel 20 is then closed by closure member 2|, carrying an elastic pressure member 22, and pressure is applied by means of lever 23 resting upon knife edge 24 and actuated by weight 25. Power is then applied to pulley 26, and impeller 21 is strongly rotated. After a period which may vary from about 30 seconds to about 3 minutes, rotation of the impeller is stopped, vessel 20 is opened, and ground mica is discharged therefrom.

Figure 3 shows a plan view of one type of impeller found effective for grinding with either of the apparatuses illustrated. The shape of this impeller, however, is not critical. A plate studded with wedges or other projections may be used, or propeller-type blades, or conventional moving parts of an ordinary hammermill. When carrying out our novel process, either batchwise or continuously, the breaking up of mica particles within the disintegration zone is accomplished primarily by forces transmitted through the mass which impose bending, torsional, and shearing stresses on the particles. Only a minor amount of grinding is accomplished through impact of agitator blades or hammers on the mica particles.

The water content of the mica undergoing grinding is a critical feature of our invention. The water content necessary has been found to be somewhat variable with the type of mica, but will be in the range of from about 15 to about 35 weight per cent. We have found that in order to obtain maximum advantages in the operation of our invention, the water content should be controlled within 5 per cent of that found to be optimum for the particular type of mica undergoing disintegration. This optimum, within the range of from 15 to 35 per cent, may be easily determined by trial.

A second critical feature of our invention is the use of restraining or confining pressure on the mica-water mass when undergoing grinding. The necessity for maintaining pressure upon the mica-water mass is believed 'to be due to the necessity --ior having a stiff mass which can absorb energy applied to it. Be that as it may, the result nevertheless is that without restraining or confining pressure on the mica-water mass the grinding efficiency is very low. We have found that the amount of restraining pressure necessary is very moderate. Pressure maintained upon the mass undergoing disintegration should amazes Higher pressures; may be used however if .dea

sired, but; such higher pressures do not; in any way increase the efiicienny of? grinding; Pres:- sures above 20 p. s.. g2, althougha operable may introduce unnecessary inconveniences and. power loss. into the: operation.

The energy for grinding the mica. in; the: disintegration: zone may be.- applied in. various manners. As mentioned:above the type: of agitator is not. critical. Any apparatus which. strongly agitates the mass may be; used- For the. sakeof convenience, rotary members are preferred. The speed oi rotation is not particularly critical. At high. speed, high: quantities-:- of; energy are con:- sumed and large quantities of mica are: dis. integrated. At low speed, less.- energy is. consumedand correspondingly smaller quantities of mica are. comminuted. The: relationships; between speed of. rotating member.. power con.- sumed, and mica. ground. are: substantially rectilinear...

That the energy applied is absorbed and cuts cientl-y utilized for disintegration. of; mica is demonstrated in the following examples of the process. In these examples the energy consumed was accurately measured and compared with standard practice from the standpoint of efiiciency. To measure the: consumed. energy Prony brake tests were made on the motors in question. Their characteristics of current vs. horsepower were determined and recorded. Thereafter when these motors were: used for grinding, the current consumption was accurately" measured on a portable am meter which, by calculation, would give the horsepower used. In this way the: process was critically examined from a. power standpoint and any misconception on the amount of grinding performed by any one machine was avoided, thereby allowing a study of process variables only. Likewise, the quality of ground. mica was determined; by the use of! standard test screens for size determinations, and a photoelectric reflection meter was used for color and sheen determinations. It is therefore believed that the following examples clearly and accurately illustrate the actual conditions and results to be expected is the use of this invention.

Example I A 1500 gram sample of scrap mica, ranging in size from minus inch to plus 6 mesh, was wetted with water sufiicient to bring the water content of the resulting mixture to 25 per cent by weight and was ground for 3 minutes under a constant restraining or confining pressure of 5.4 p. s. i. g. The apparatus used was similar to that hourwasrobtaineda The-color readingeofith-ism-ica was 65 per cent of standard magnesium oxide, measured through a. greerrfilter. This efficiency compares with an estimated. 5'. pounds per horse.-

power-hour in commercial. plantpractice: using muller-grinding processes- The-minus- 3'25-mesh fraction of. mica. had. a weight of 10.1 pounds. per cubic foot as measured ona Scott volu-meter, using a. l-cubic-inch box. The color, sheen-,, bulk density,v feeL. and adherence of the ground mica obtained were fully equal to similar properties of mica ground by the muller process- Example. I! I The process of Example I was repeated many times, using the same apparatus, various types. of feed, various water contents, various times of agitation, and various pressures. All ground mica produced was of commercial water-ground quality except in Test 14. The following results aretypical.

Bounds i nus 'Prcssurc i. POI centv d Timeof meshlest Fypc Watprin Agitator, r pounds a agitation, mica number fccd feed .RU. rminutes, peiznsq. pm

' duccd.

per hp.-ln

A 50 2,480 5.- i 2.95 A 0 1, 550 0. 1. 80 A 25 1 780 1 e i s. 7. 01 a 25 325- e 1 5. 9.05 A 25 325 s i 0.30 A 25 325 3 1s 10. A 325. i 3' 23. 8. A 25 325' 9 1a. 4.14 25'. 325 r 4 IS. 20100 B 25 '325: 3 10. 14.70 0- 25 325 3' 1 0'. 12. 80 B 25 i 325' l. 10'. 241.50 0 25 32s 1 10. 24.50 D 35 325 3 5; 10; 00 n 30 325 2 5;. 13.70 D 25 325 a 5. 13. E 35 325 3 5-. 13:30 30 325 a- 5. 13.70 E 25 325 3. 5. 1.4..80

1 Type feed: A, mica scrap, pegmatite type, minus 3 mcsh, plus 8-mesl1; B, mica scrap, pegmatite type, minus it-inch, plus fi-mesh; C, mica scrap, pegmatite type, minus 6mesh, plus Iii-mesh; D, Clarksville schist, ground 2 minutes in rod mill and concentrated by flotation; E, Olarksville schist, ground 10 minutes in rod mill and concentrated by flotation.

Ewample III A continuous-feed type apparatus similar to that shown in Figure 1 was constructed and many samples of mica were ground therein at various feed rates, various retention times in the disintegration zone, and with varying quantities of water in the feed. All ground mica produced was of commercial water-ground quality. Typical results obtained are set forth in the table below.

shown in Figure 2. The resulting ground mica was removed from. the apparatus and was separated into a minus 325-mesh portion and a plus 325-mesh portion. An efliciency factor of 16.3

It will be 1 observed that very high efficiency hitherto unobtainable in the grinding of mica was obtained when the mica was disintegrated in the presence of from 15 to 35 weight per cent pounds of minus 325-mesh mica per horsepowerwater and under constant pressure.

Having described our invention and explained its operation, we claim:

1. A process for producing wet-ground mica which comprises adding water to coarse mica in such proportions that the resulting stiii, nonfiuid mass contains from about 15 to about 35 weight per cent of water; introducing such moistened mica into a disintegrating zone; therein maintaining pressure of from about 3 to about 11 p. s. i. g. on said moistened mica while agitating the mica for a period of from about minute to about 3 minutes, thereby disintegrating coarse particles of mica into finer particles; and withdrawing disintegrated mica from said disintegration zone.

2. A continuous process for producing wetground mica which comprises adding water to coarse mica in such proportions that the resulting stifi, moistened mass contains from about 15 to about 35 weight per cent of water; continuously forcing such moistened mica into a disintegration zone; therein maintaining pressure of from about 3 to about 11 p. s. i. g. on said moistened mica; continuously agitating said moistened mica in said disintegration zone, thereby disintegrating coarse particles of mica into finer particles; continuously withdrawing disintegrated mica from said disintegration zone; controlling the rate at which mica is withdrawn from said disintegration zone so that the time of retention therein is in the range of from /2 minute to about 3 minutes; separating the withdrawn mica into two fractions, namely, a fine fraction having a bulk density of not more than 12 pounds per cubic foot and a color value in the range from 65 to 70 when measured on a Photovolt reflection meter, and a coarser fraction; and recycling said coarser fraction.

3. A process for producing wet-ground mica which comprises adding water to coarse mica in such proportions that the resulting stiff, nonfluid mica mass contains from about 15 to about 35 weight percent of water; introducing such moist-L ened mica mass into a disintegration zone; therein maintaining a confining pressure of from 2 to about 20 p. s. i. g. on the mica mass while strongly agitating the mica for a period from about /2 minute to about 3 minutes, thereby disintegrating coarse mica particles into finer particles; and withdrawing disintegrated mica from said disin tegration zone.

4. A continuous process for producing'wetground mica which comprises adding water to coarse mica in such proportions that the resulting stifl", nonfluid moistened mica mass contains from about 15 to about 35-weight per cent of water; continuously forcing such moistened mica into a disintegrating zone; therein maintaining confining pressure of from 2 to about 20 p. s. i. g. on said moistened mica; continuously-strongly agitating said moistened mica in said disinte gration zone, thereby disintegrating coarse par ticles of mica into finer -particles; continuously withdrawing disintegrated mica from said disintegration zone; and controlling the rate at which mica is withdrawn from said disintegration zone so that the time of retention therein is in the range from about /2 minute to about 3 minutes.

WILLIAM T. MoDANIEL, JR. JOHN R. LE GRAND.

REFERENCES CITED The following references are of, record in the file of this patent:

Urumn STATES PATENTS OTHER REFERENCES Bureau of Mines Bulletin No. 2798, Mar. 1927, 252-378, The Use of Flocculating Reagents for the Recovery of Fine Mica. W. M. Myers. 

